A string is stretcbed and secured on the $x$ -axis at $x=0$ and $x=\pi$ for $t>0$. If the transverse vibrations take place in a medium that imparts a resistance proportional to the instantaneous velocity, then the wave equation takes on the form
$$
\frac{\partial^{2} u}{\partial x^{2}}=\frac{\partial^{2} u}{\partial t^{2}}+2 \beta \frac{\partial u}{\partial t}, \quad 0<\beta<1, \quad t>0
$$
Find the displacement $u(x, t)$ if the string starts from rest from the initial displacement $f(x)$.